Automatic changeover manifolds are extensively utilized by the user of various gases where the supply is from cylinders or banks of cylinders and where the requirement is such that the flow must continue uninterrupted when one of the cylinders or banks becomes exhausted.
Examples of such operations in common usage are welding operations and breathing and anaesthetic gas flows in hospital environments. Most automatic changeover manifolds (ACM) utilize specialized changeover valves of the diaphragm type or regulators using rubberized diaphragms, set to different pressures, so that changeovers can occur and specialized 4-way valves configured to semi-automatic operation. Such conventional systems are not capable of controlling the flow of cold cryogenic liquified gas.
A search with respect to the present invention has been conducted in the facilities of the U.S. Patent and Trademark Office and the following U.S. Patents have been noted as the most relevant:
______________________________________ 3,001,541 2,714,292 2,547,823 2,402,187 4,341,234 4,597,406 3,583,421 3,013,573 ______________________________________
Of these references, U.S. Pat. No. 2,402,187 is considered to be the most pertinent as is discussed in detail below. None of the cited prior art describes the handling of cold cryogenic liquid gases but generally disclose systems for maintaining the uninterrupted flow of gases.
U.S. Pat. Nos. 2,547,823 and 3,001,541 specifically illustrate the use of diaphragm-controlled valves. U.S. Pat. No. 2,714,292 requires a manual reset when an exhausted supply is replenished. U.S. Pat. No. 3,013,573 describes the control of flow of chemicals to a chemical stabilizing operation using "conventional pressure switches" i.e. diaphragmed switches. U.S. Pat. No. 3,583,421 describes a particular valve structure for use in a hospital anaesthetic supply system. U.S. Pat. No. 4,341,234 describes an acetylene supply system which is adapted to achieve an improved gas utilization. U.S. Pat. No. 4,597,406 describes a system for delivering high purity gas at constant pressure using a particular switching control system.
U.S. Pat. No. 2,402,187, the closest known art, describes an automatic control system for four acetylene generators, arranged in two independent groups of two generators each. The electrical circuit is divided into two independent and identical circuits, so that description of the operation of one pair of the generators only is necessary.
As the supply of acetylene from one generator declines sufficiently that the pressure produced falls below a predetermined minimum value, the pressure switch associated with that flow line is activated and closes a pair of contacts, which causes an alarm to sound and a visual signal to appear on the control panel to indicate that the generator is inoperable and requires recharging. Closing of the contacts by the pressure switch also energizes one coil of a two-coil relay, which then opens normally-closed switch contacts and closes normally-open switch contacts. This activity causes the motor-driven valve associated with the first generator feed line to close and the motor-driven valve associated with the second generator feed line to open, so that the second generator comes on-stream.
The opening of the normally-closed switch contacts and the closing of the normally-open switch contacts also causes a visual indicator that the one generator is on-line to be extinguished and a visual indicator that the other generator is now on-line to be lit. The alarm is disabled by a manual reset switch. The first generator is recharged and, when the second generator becomes exhausted, the procedure is reversed.
It is evident, therefore, that the two-coil relay and associated contacts act as an interconnected control mechanism for the flow valves, constructed and arranged such that when either generation unit is on-stream, the other is cut off.
A draw-back to this prior art system, and one overcome in the present invention, is that, if both generators are inoperative at the same time, so that both pressure switches are closed, it is necessary to open manually a push button to prevent recycling of the relay. Otherwise, the circuits through the relay coils will be alternately made and broken in continuous cycles as the switch contacts are alternately opened and closed. In the present invention, in the absence of gas flow, the system assumes a stand-by mode, without the necessity for manual intervention.